A novel C-type lectin with triple carbohydrate recognition domains has critical roles for the hard tick Haemaphysalis longicornis against Gram-negative bacteria.

C-type lectins (CLecs) play an important role in innate immunity against invaders. In this study, a novel CLec was identified from Haemaphysalis longicornis ticks (HlCLec). HlCLec contains a signal peptide and a transmembrane region. Interestingly, HlCLec possesses three dissimilar carbohydrate recognition domains (CRDs). Each CRD contains the mutated motif of Ca(2+)-binding site 2. HlCLec mRNA was up-regulated during blood feeding, and had highest expression in the midgut and ovary. HlCLec localization was also confirmed by immunofluorescent antibody test (IFAT). HlCLec was found on the cell membrane and basal lamina of midgut and ovary. In addition, the recombinant HlCLec and individual CRDs demonstrated direct binding activity to Escherichia coli and Staphylococcus aureus; however, no growth inhibition activity was observed. Furthermore, E. coli injection after silencing of HlCLec caused drastic reduction in survival rate of ticks. These results strongly suggest the key role of HlCLec in tick innate immunity against Gram-negative bacteria.